Foundation part of a structural system that supports and anchors the superstructure of a building and transmits its loads directly to the earth. To prevent damage from repeated freeze-thaw cycles, the bottom of the foundation must be below the frost line. The foundations of low-rise residential buildings are nearly all supported on spread footings, wide bases (usually of concrete) that support walls or piers and distribute the load over a greater area. A concrete grade beam supported by isolated footings, piers, or piles may be placed at ground level, especially in a building without a basement, to support the exterior wall. Spread footings are also used—in greatly enlarged form—for high-rise buildings. Other systems for supporting heavy loads include piles, concrete caisson columns, and building directly on exposed rock. In yielding soil, a floating foundation—consisting of rigid, boxlike structures set at such a depth that the weight of the soil removed to place it equals the weight of the construction supported—may be used.